As one skilled in the aeronautical art appreciates the integral bladed rotor formed from superalloy materials which sometimes is referred to as a blisk or IBR is a relatively new concept for fabricating axial flow turbine and/or compressor rotors utilized in the gas turbine engines. Heretofore, the blades and disk of the rotors were individually formed and the blades were typically attached to the rim of the disk with a well known dovetail or fir tree root attachment assembly. The IBRs are formed by forging the blades and disk into a rough finished form and machining the airfoil of the blades in the final configuration. The blades of the IBR are either forged integrally with the disk or the blades are metallurgically bonded to the disk.
Examples of methods of fabricating and repairing IBRs are disclosed in U.S. Pat. No. 4,479,273 granted to Miller et al on Oct. 30, 1984 and entitled "Process For Fabricating Integrally Bladed Bimettallic Rotors", U.S. Pat. No. 5,113,583 granted to Jenkel et al on May 19, 1992 entitled "Integrally Bladed Rotor Fabrication" and U.S. Pat. No. 5,109,606 granted to DeMichael et al on May 5, 1992, all being assigned to the assignee common to this patent application and being incorporated herein by reference.
Whether the blades and disks are integrally formed or the blades are metallurgically attached to the disk or for that matter regardless of the method of forming the IBR into the rough or unfinished form, problems have arisen during the final machining operation. Namely, because of the metal utilized and the thinness, of the airfoil section of the blades, the structure has the propensity of flexing in the machining process. Typically, the airfoils are machined to the final configuration by a computerized milling machine. The cutting tool or tools in a multiple cutting operation exert a force on the blades being machined which has a tendency of causing the blades to deflect. This deflection is acerbated when the cutter(s) moves in a radially outward direction. Obviously, any deflection during the machining operation will adversely affect the final dimensions of the airfoil and consequently, the tolerances required are adversely affected. It is imperative that the tolerances of these engine components, particularly when used in aircraft powered by gas turbine engines, are held to a minimum.
It is well known and common practice for these machining operation to cast into the cavity or spaces between each of the blades a low temperature casting material such as a plaster or plastic to add rigidity to the blades and reduce and minimize the deflection of the airfoils during the machining process. While this practice is acceptable in certain applications it is not acceptable in the gas turbine engine applications.
This invention obviates the problems alluded to in the above paragraphs by incorporating a fixture that enhances the rigidity of the airfoils and hence, minimizes or eliminates the deflections heretofore occasioned by the finished machining operation. A ring fixture surrounding and spaced from the tips of the airfoil is fitted with a plurality of circumferentially spaced pins that are inserted into the space between blades. A suitable plaster or plastic material is cast into the space and the combination of the ring, pins and cast material adequately support the rotor during the final cutting of the airfoil configuration. In accordance with the method taught in this invention, the pins are discretely formed with a large diameter head, a threaded shank portion and a tapered shank portion. The pin is secured to the ring and the tapered shank is embedded into the cast material. Alternate spaces are outfitted with the pins in preparation of the first machining operation. The cast material is poured between all of the blades and then cured. The entire assembly including the hardened cast material and the ring/pin arrangement are then mounted on the milling machine for the cutting of the surfaces of the airfoil where no pins are located. The pins are then removed and pins are installed in between blades. The unit is then casted with a plastic or plaster medium, curd so that the solidfified medium supports the fixture to the IBR and then returned to the milling machine where the remaining airfoil surfaces are cut to the final configuration. The fixture and rotor with the finished machined airfoils are removed from the cutting machine and heated to a temperature to melt the cast material which inherently releases the pins and the fixture from the IBR.
In an actual test of machining the IBR into its final configuration by utilizing this invention, it was found that an IBR made from a titanium alloy allowed the machining operation to feed at 5.25 inches per minute (ipm) in the machining operation. The feed rate of the heretofore machining operation was 3.0 ipm. This resulted in a 75% increase in feed rate. The machining of the finished IBR required 168 hours which was a 11.2% reduction in time. Obviously, this resulted in a considerable savings in machining time. Using a shop rate at $100 an hour, the use of this invention compared to heretofore known methods resulted in a savings of $16,800.00 per IBR. Also, of importance, was the fact that the structural integrity during the machining operation remained in tact so that the end product achieved its specified tolerances.